Such lamps and ballasts have been known per se for a long time. Use is made in one group of appliances of a so called PTC element (a resistor with a decidedly positive temperature coefficient) for stipulating a preheating time when such a lamp is restarted. The PTC element is heated up during preheating by a current and terminates the preheating operation by increasing its electric resistance.
The control of the converters, in particular of the switching transistors used therein, can be performed, on the one hand, by feedback, in which case a so called self-excited converter is spoken of. On the other hand, it is also known to control converters externally by means of a sequential control system and, in the process, particularly to influence the operating frequency of the converter, for example in order to control the lamp current in continuous operation.
As a rule, the ballasts are designed for operating on an ac voltage supply system. A rectifier is used to generate an intermediate circuit dc voltage that is used to supply a converter which, in turn, generates a supply of power of higher frequency than the system frequency for the purpose of operating the lamp.
An important property of such ballasts is the way in which power is drawn from the ac voltage supply system. When the rectifier charges an intermediate circuit storage capacitor, abrupt charging processes come about in the intermediate circuit storage capacitor without any further measures when the instantaneous system voltage is above the capacitor voltage. This generates line current harmonics and causes a poor power factor.
There are various possibilities for improving the power factor, that is to say for reducing the line current harmonics. The corresponding properties of electronic ballasts are also covered in part by regulations, for example IEC1000-3-2. In addition to dedicated converters for charging the intermediate circuit storage capacitor (or, more generally, main energy store) from the rectified system voltage, so called pump circuits also come into consideration. The latter require a comparatively low outlay on circuitry.
It is inherent in the topology of a pump circuit that the power rectifier is coupled to the intermediate circuit storage capacitor via at least one electronic pump switch. This results in a pump node between the power rectifier and the electronic pump switch. Said pump node is coupled to the converter output via a pump network. The pump network can include components that at the same time can be assigned to a matching network for coupling the lamp to the converter output. The principle of the pump circuit consists in withdrawing energy from the rectified system voltage via the pump node during a half period of the converter frequency, and buffering it in the pump network. In the subsequent half period, the buffered energy is fed to the intermediate circuit storage capacitor via the electronic pump switch.
Energy is consequently withdrawn from the rectified supply voltage in step with the converter frequency. In general, the electronic ballast includes filter circuits that suppress spectral components of the line current in the region of the converter frequency and above. The pump circuit or circuits can be designed such that the line current harmonics comply with the abovementioned regulations or other requirements.
As regards pump circuits, reference may otherwise be made to the prior art, specifically in particular to the applications DE 103 03 276.2 and DE 103 03 277.0 from the same applicant and to the citations there.